1. Field of the Invention
The present invention relates to an image sensor and to a method of forming an image sensor, and in particular to an image sensor comprising an array of pixels.
2. Discussion of the Related Art
Monolithic image sensors comprise photodiodes and transistors formed in a silicon substrate. More specifically, such image sensors comprise an array of pixels each having a pinned photodiode coupled to a sensing node by a transfer transistor. A charge accumulated by the photodiode during an integration period can be transferred to the sensing node via the transfer transistor.
Reading the voltage stored at the sensing node is performed using read circuitry, generally comprising a source follower transistor, having it gate coupled to the sensing node. Furthermore, a reset transistor is also provided coupled to the source, allowing the voltage of the source to be reset after each read. To reduce the number of components, the read circuitry is often shared by more than one photodiode.
The sensitivity and thus quality of the image sensor are, to some extent, determined by the charge holding capacity of each photodiode of the image sensor. In particular, if the photodiode becomes saturated during an integration period and can no longer store more charge, this results in a reduction in image quality.